For some applications, it is desirable to have a liquid crystal variable transmission filter that can transmit at least 50% of unpolarized light incident on it. Such high transmittance is not possible with traditional liquid crystal variable transmission filters with high-durability input and output polarizers because the high amount of light absorption by these polarizers allows the variable transmission filter to transmit only 18%-20% of unpolarized light. A liquid crystal variable transmission filter design that does not require polarizers could achieve at least 50% transmission, or f/1. Such a design should also have minimal color shift over its operating range, as well as minimal variation of the transmitted luminance over a range of incident polar angles especially about the normal incidence direction.
A guest-host liquid crystal camera iris represented in FIGS. 6A and 6B of U.S. Patent Application Pub. No. US 2012/0242924 has two homogeneously aligned guest-host liquid crystal cells placed in optical series and oriented with their surface alignment directions at 90°. (These two guest-host cells configured with a 90°-surface alignment direction orientation are referred to as two crossed guest-host cells.) While it is theoretically possible that this camera iris could transmit 50% of unpolarized light at normal incidence, there is an unacceptably large viewing angle dependence of the transmittance. Such angular dependent transmittance would result in a non-uniform and asymmetric perception of light intensity, depending upon the viewing angle of the transmitted light reaching the eye of an observer or a light-sensitive area of a recording medium in a camera.